1. Field of the Invention
The present invention relates to a radiation imaging apparatus and an imaging system.
2. Description of the Related Art
A radiation imaging apparatus including a pixel array obtained by arranging a plurality of pixels including transistors which supply desired potentials to conversion elements for converting X-rays or light into charges is required to reset the charge generated by the dark current generated in each of the plurality of pixels before irradiation with radiation. For this reason, a radiation imaging apparatus of this type is designed to perform the reset operation of periodically resetting charges in a plurality of pixels by periodically and sequentially setting the transistors of a plurality of pixels in a conductive state to reset voltages applied to conversion elements.
Japanese Patent Laid-Open No. 2007-151761 discloses a radiation imaging apparatus which resets voltages applied to conversion elements by using transfer switch elements for transferring signals corresponding to the charges generated by conversion elements as transistors to signal lines and to a circuit for processing the signals from the signal lines. This literature also discloses that the radiation imaging apparatus detects the irradiation of X-rays and interrupts reset operation in response to the irradiation of X-rays. Interrupting reset operation will shift to the accumulation operation of setting all the transistors in a non-conductive state to cause the pixels to accumulate charges corresponding to the irradiation of X-rays. The literature also discloses that after a predetermined period of time (accumulation time) has elapsed since the start of the accumulation operation, the apparatus starts actual read operation corresponding to the readout operation of sequentially reading out the charges as pixel signals. According to Japanese Patent Laid-Open No. 2007-151761, this method shortens the time between the irradiation of X-rays and the start of the operation of reading out image signals and also suppresses the occurrence of artifacts caused by reset operation.
Japanese Patent Laid-Open No. 2010-268171 discloses a technique of making the duration of the conductive state of each transistor in reset operation longer than the conducting time of each transistor in readout operation. This improves the reset efficiency.
Note that interrupting reset operation periodically performed in accordance with the irradiation of X-rays in the above manner will make the pixel array include the first block which has been reset before the interruption and the second block which has not been reset before the interruption. The first and second blocks differ in the accumulation time during which each transistor is maintained in a non-conductive state. This causes differences in noise signal value based on the dark current generated in each pixel during the accumulation time. This can cause a large density difference in the boundary portion between the first image region corresponding to the first block and the second image region corresponding to the second block. This will impair the reliability of a radiation image.